Medical devices that deliver drugs through the skin for absorption into the body have been known for some time. For example, U.S. Pat. No. 3,249,109 describes a two-layer topical dressing that consists of an adhesive base made of drug-containing hydrated gelatin with a fabric backing layer. This type of device delivers a varying amount of drug to the skin and the rate of absorption is determined by the release rate of drug from the device which decreases as a function of time of application and permeability of the skin at the administration site. In order to transdermally deliver drugs having a relatively narrow therapeutic range, and for which such wide variations could not be tolerated, "system-controlled" delivery devices, which deliver drugs transdermally at rates which are controlled primarily by the delivery device, were developed to reduce or eliminate the variations in delivery rate associated the uncontrolled devices described above. For example, U.S. Pat. No. 3,598,122 describes a multilayer adhesive bandage for delivering drugs into the systemic circulation formed of a backing layer, a drug reservoir layer, a contact adhesive layer, and includes means for metering the rate at which the drug is released to the skin. Other representative system controlled transdermal drug delivery devices are described in U.S. Pat. Nos. 3,797,494 and 4,379,454, the latter of which teaches controlling the rate at which a drug is absorbed through the skin by controlling the rate at which a permeation enhancer for the drug is delivered to the skin. (All of the aforementioned U.S. patents are incorporated herein by reference.) In addition, Black, "Transdermal Drug Delivery systems", U.S. Pharmacist, November, 1982, pp. 49-78, provides additional background information regarding commercially available transdermal drug delivery systems and a reasonably representative summary of the factors involved in percutaneous absorption of drugs may be found in Aritz, et al., "Studies on Percutaneous Absorption of Drugs", Chem. Phar. Bull., Vol. 18, 1970, pp. 1045-1049; Idson, "Percutaneous Absorption", J. Phar. Sci., Vol. 64, No. 6, pp. 910-922; and Cooney, Advances in Biomedical Engineering, Part 1, Chapter 6, "Drug Permeation Through Skin: Controlled Delivery for Topical Systemic Therapy", Marcel Dekker, Inc., New York and Basel 1980, pp. 305-318.
Although the transdermal drug delivery route is rapidly becoming a preferred delivery route for a wide variety of drugs, transdermal delivery is not without its problems. For example, transdermal systems generally have a relatively long lag time between the time the device is applied to the skin and the time that therapeutic levels are achieved in the blood. This is because the transfer of the therapeutic agent from the device into the bloodstream is a diffusional process and requires the necessary concentration gradient to be established between the device and the internal surfaces of the skin. Attempts to decrease the lag time have been proposed and include a "pulse" dosage of the drug in the adhesive layer in contact with the skin in order to initially saturate the skin binding sites so that delivery into the systemic circulation can begin sooner and treatment of the skin with permeation enhancers, either prior to administration of the device or concurrently with the drug administration. (See for example, U.S. Pat. No. 4,031,894 which is incorporated herein by reference.)
A more fundamental limitation of the rate controlled and non-rate controlled devices of the prior art is that they are designed to deliver a drug into the systemic circulation at either a substantially constant rate throughout a substantial portion of the administration period or at a generally declining rate with time of administration. In many circumstances it would be extremely desirable to achieve and maintain an initially high blood level of a drug for a significant portion of the administration period and therafter maintain a lower, but still constant, blood level for the remainder of the administration period. This type of dosage regime would be desirable in those instances in which a rapid onset of therapeutic effect is desired. Rapid onset could be obtained from a high initial blood level which could not be tolerated for the entire administration period due to the undesirable side effects that could result from the maintenance of the high blood level. Thus drug blood levels for the remainder of the administration period would have to be reduced to a lower, but still constant, therapeutic level. Such a dosage regime would also be desirable in administration of a drug which may create a tolerance to the therapeutic effect if the drug is administered at a constant continuous rate. In such circumstances for example, the initial high blood levels may be more effective when followed by a lower maintenance level than if the blood levels were maintained either at the higher or the lower level throughout the entire administration period. Nitroglycerin, for example, could be delivered in such a regime.
It is therefore a primary object of this invention to provide a transdermal drug delivery device adapted to administer a drug throughout a predetermined administration period in which the drug is delivered at a first, higher, substantially constant rate for a significant portion of the delivery period followed by a second, and lower, substantially constant delivery rate for the remainder of the administration period. Such a device is particularly suitable for delivering drugs which are capable of permeating through normal skin at rates which produce therapeutic doses from reasonably sized devices without the use of permeation enhancers. It also contemplated that the device could be employed to deliver drugs which are less permeable if the skin at the delivery site is pretreated to increase its permeability by perforating, stripping, abrading or chemically treating the stratum corneum.
It is another object of this invention to provide a pulsatile transdermal drug delivery device.
It is another object of the device to provide a transdermal drug delivery device capable of delivering a drug into the systemic circulation substantially constant delivery rates within sequential predetermined portions of predetermined administration period.
It is another object of this invention to provide a method for the pulsatile transdermal delivery of a drug.